Municipal wastewater

Wastewater is treated in sewage treatment plants. Biological and chemical processes as well as mechanical ones are used here. Most of the clogging material is caught by the screening system. Heavy matter, such as particles of sand that are being carried along, should settle here. The final station of the mechanical treatment stage is the primary settlement tank. All the lighter materials that are still in the wastewater and which have not been removed in the grit chamber settle to the floor of this tank to form socalled raw sludge. While the pretreated water is forwarded to the aeration tank the raw sludge is conveyed to the digesters. The biological treatment of the wastewater takes place in the aeration tank. Before the wastewater gets to this tank the wastewater is mixed with activated sludge. This sludge contains countless microorganisms (e.g. bacteria) that are able to break down the colloidal, organic contaminants dissolved in the wastewater. The activated sludge settles in the secondary settlement tank and collects at the bottom. The collected sludge is either removed and taken back to the aeration tank as return activated sludge or the collected sludge is conveyed to the digesters as surplus activated sludge. Digestion is the last station of the biological treatment stage. The sludge is stabilized in the digester. Stabilization refers to the most advanced anaerobic degradation of organic compounds with the aid of specific bacteria. These bacteria convert the organic components of the anaerobic digested sludge into biogas.